This 2011 United States Marshals Service mug shot shows former oganized crime figure, suspect James "Whitey Bulger. Bulger was sentenced to life in prison on November 14, 2013. Judge Denise J. Casper also ordered Bulger to pay 19.5 million USD in restitution.

Boston Mob Boss 'Whitey' Bulger Appeals His Conviction

Former Boston crime boss James "Whitey" Bulger is appealing the federal conviction that sent him to prison for life last year, arguing that he was unable to fully present his defense. Bulger, who's 84, was convicted and sentenced on racketeering charges that tied him to 11 murders and other gangland crimes from the 1970s and '80s. In an appeal filed Thursday in the 1st U.S. Circuit Court of Appeals, Bulger's lawyers Henry Brennan and James Budreau argue he was "deprived" of a critical defense in his trial: that a now-dead federal prosecutor had given him immunity from his crimes.

During the trial last year, Judge Denise Casper ruled Bulger couldn't raise the immunity claim because he offered no hard evidence to support it. The judge also said prosecutor Jeremiah O'Sullivan, who died in 2009, didn't have authority to grant such immunity.

This "constitutional error" affected the fairness of Bulger's trial and must result in reversal, his lawyers wrote in their appeal. "If Mr. Bulger had been permitted to testify about his immunity defense in his own words, then the jury would have the opportunity to weigh his credibility with that of the government's witnesses," they wrote. "... His testimony alone could have made a difference in the verdict."